Not voting is a choice and allows voter apathy to play a role in deciding elections. If an area is a majority Democrat but most of the voters don't care enough to vote then a motivated minority party can put boots on the ground and get their candidate into office. With mandatory voting that would never happen as the majority incumbent would always win thanks to receiving all those votes cast out of obligation rather than intent. That aggravates the most difficult part of democracy which is giving people with unpopular yet valid opinions a voice in the conversation.
Now, if we had proportional representation...
The population does not feel represented by the politicians, yet we have to choose one of them every election. Buying votes /manipulating electors in this scenario looks really easy..
The people in power almost always badly represent the common interest; I hope this changes in my lifetime.
Australia is the country most like the US on the list. I don't know enough about the Central American countries on the list to judge their internal politics. The unenforced list includes Belgium and France.